Comments

Is this an old controller? The 3 fans in these controllers can be quite noisy. What voltage are you inputting and charging batteries at? The higher the differential the harder the controller will be working. If it's got some miles on it you may need to replace a failing fan.

Are you young with good hearing? There may be radio frequency that younger ears can hear which go unnoticed by older people who have lost the upper frequency detection, or perhaps your hearing is more acute, if of course it's not the fans.

What is the voltage into the classic from the array, and what is the battery voltage when you hear the whine?

Mine make a high pitched sound when Varray is close to Vbatt (eg. Varray 54v, Vbatt 52v), typically when they try to bulk in gloomy weather. Normal Varray is ~100v in decent light. IIRC, there's a setting you can disable (might be arc fault) to minimize the noise if it's driving you nuts, but it isn't hurting anything.

The classic has 2 normal cooling fans and a 3rd "Turbo" fan that kicks in if the others are not enough, Since you are overpaneled, I expect it to be running hot and needing the fans,They also use a switcher supply, which operates in the ultrasonic range, and you may be hearing that, or sub-harmonics. (You are not hearing RF, unless you have implants that allow for that.)

At 9:30am 110v coming in and it's at Bulk MPPT with no noises. I'll check again in the afternoon when the fans are loud.I have bad hearing so any funky frequency sounds are not heard by me. The original setup was 4 250w panels. This spring we added 4 265w panels. The 250's are wired together and the 265's are wired together for 2 sets coming in to the controller. This is how Northern AZ Wind & Solar told me to wire and said the controller could handle it.As a side not, the controller is very warm to the touch when making the noises.

It is just running hot, it's converting the 110 volts coming in, to charging voltage for the 24 volt battery bank. Usually suggest about double the battery bank voltage.

Often the 200 and 250's are sold due to long wire runs so people can run higher voltage and save the cost of heavy wire for long distances. Since you have 2 strings of panels it also saves you from having to buy a combiner box.

The 250 is designed to handle the higher voltage so I don't think it's a real problem just and annoyance. Is this a new sound? I know they had an issue with very early software and the turbo fan kicking in before it was needed (or at least that's my memory of the situation) I would suggest finding your firmware version and contacting Midnite. They have very good customer service!

Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites, Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.